1. Field of the Invention
The present invention relates to the papermaking arts. More specifically, the present invention relates to forming fabrics for the forming section of a paper machine.
2. Description of the Prior Art
During the papermaking process, a cellulosic fibrous web is formed by depositing a fibrous slurry, that is, an aqueous dispersion of cellulose fibers, onto a moving forming fabric in the forming section of a paper machine. A large amount of water is drained from the slurry through the forming fabric, leaving the cellulosic fibrous web on the surface of the forming fabric.
The newly formed cellulosic fibrous web proceeds from the forming section to a press section, which includes a series of press nips. The cellulosic fibrous web passes through the press nips supported by a press fabric, or, as is often the case, between two such press fabrics. In the press nips, the cellulosic fibrous web is subjected to compressive forces which squeeze water therefrom, and which adhere the cellulosic fibers in the web to one another to turn the cellulosic fibrous web into a paper sheet. The water is accepted by the press fabric or fabrics and, ideally, does not return to the paper sheet.
The paper sheet finally proceeds to a dryer section, which includes at least one series of rotatable dryer drums or cylinders, which are internally heated by steam. The newly formed paper sheet is directed in a serpentine path sequentially around each in the series of drums by a dryer fabric, which holds the paper sheet closely against the surfaces of the drums. The heated drums reduce the water content of the paper sheet to a desirable level through evaporation.
It should be appreciated that the forming, press and dryer fabrics all take the form of endless loops on the paper machine and function in the manner of conveyors. It should further be appreciated that paper manufacture is a continuous process which proceeds at considerable speeds. That is to say, the fibrous slurry is continuously deposited onto the forming fabric in the forming section, while a newly manufactured paper sheet is continuously wound onto rolls after it exits from the dryer section.
Press fabrics also participate in the finishing of the surface of the paper sheet. That is, press fabrics are designed to have smooth surfaces and uniformly resilient structures, so that, in the course of passing through the press nips, a smooth, mark-free surface is imparted to the paper.
Press fabrics accept the large quantities of water extracted from the wet paper in the press nip. In order to fill this function, there literally must be space, commonly referred to as void volume, within the press fabric for the water to go, and the fabric must have adequate permeability to water for its entire useful life. Finally, press fabrics must be able to prevent the water accepted from the wet paper from returning to and rewetting the paper upon exit from the press nip.
Woven fabrics take many different forms. For example, they may be woven endless, or flat woven and subsequently rendered into endless form with a seam.
The present invention relates specifically to the forming fabrics used in the forming section. Forming fabrics play a critical role during the paper manufacturing process. One of its functions, as implied above, is to form and convey the paper product being manufactured to the press section.
However, forming fabrics also need to address water removal and sheet formation issues. That is, forming fabrics are designed to allow water to pass through (i.e. control the rate of drainage) while at the same time prevent fiber and other solids from passing through with the water. If drainage occurs too rapidly or too slowly, the sheet quality and machine efficiency suffers. To control drainage, the space within the forming fabric for the water to drain, commonly referred to as void volume, must be properly designed.
Contemporary forming fabrics are produced in a wide variety of styles designed to meet the requirements of the paper machines on which they are installed for the paper grades being manufactured. Generally, they comprise a base fabric woven from monofilament, plied monofilament, multifilament or plied multifilament yarns, and may be single-layered or multi-layered. The yarns are typically extruded from any one of several synthetic polymeric resins, such as polyamide and polyester resins, used for this purpose by those of ordinary skill in the paper machine clothing arts.
This invention describes a fabric that breaks up undesirable drainage marks in forming fabrics that use pairs of integral machine direction (MD) binding yarns to hold multi layer fabrics together. In the prior art, the MD yarns may be comprised of as little as 10% binders or as many as 100% binders. References describing fabrics with paired integral MD yarns are U.S. Pat. No. 4,501,303 (the “Österberg” patent) where these pairs are an integral part of the top weave but act as binding yarns on the bottom weave, U.S. Pat. No. 5,152,326 (the “Vöhringer” patent) which focuses on these pairs making up at least 10% of the MD yarns and are integral parts of both the top and bottom weave and U.S. Pat. No. 4,605,585 (the “Johansson” patent) which has 100% of the MD yarns made up of these pairs. The disadvantages of Österberg, Vöhringer and Johansson are either strong topside diagonals or strong drainage diagonals formed from how the yarns cross each other and align in the woven cloth. (The Vöhringer patent will be described in detail later.)
FIG. 3 is a forming side view of a fabric woven in accordance with the teachings of the Johansson patent. The Johansson patent describes a double layer forming fabric with one warp system that is made of pairs of MD yarns that alternate making the top and bottom side of the cloth. While one of the pairs is weaving the topside weave pattern the other is weaving the bottom side weave pattern. The pairs then cross between the top and bottom sides of the cloth so that the yarn weaving the topside of the weave pattern is now weaving the bottom side and vice versa. As described by Johansson, the pairs make up 100% of the MD yarns. In FIG. 3, the crossover points 300, where the two yarns in a pair cross each other, are circled. Notice how the crossover points line up to make a strong topographic diagonal pattern. The diagonal line 310 highlights a sequence of crossover points along the same diagonal pattern. Unfortunately, when using 100% paired integral MD yarns, it is impossible to spread the crossover points far enough apart to eliminate this strong topographical defect formed by the crossover points lining up in a diagonal pattern.
The design of forming fabrics additionally involves a compromise between the desired fiber support and fabric stability. A fine mesh fabric may provide the desired paper surface properties, but such design may lack the desired stability resulting in a short fabric life. By contrast, coarse mesh fabrics provide stability and long life at the expense of fiber support. To minimize the design tradeoff and optimize both support and stability, multi-layer fabrics were developed. For example, in double and triple layer fabrics, the forming side is designed for support while the wear side is designed for stability.
In addition, triple layer designs allow the forming surface of the fabric to be woven independently of the wear surface. Because of this independence, triple layer designs can provide a high level of fiber support and an optimum internal void volume. Thus, triple layers may provide significant improvement in drainage over single and double layer designs.
Essentially, triple layer fabrics consist of two fabrics, the forming layer and the wear layer, held together by binding yarns. The binding is extremely important to the overall integrity of the fabric. One problem with triple layer fabrics has been relative slippage between the two layers which breaks down the fabric over time. In addition, the binding yarns can disrupt the structure of the forming layer resulting in marking of the paper.
The present invention describes a paired warp triple-layer fabric where like adjacent yarns from adjacent pairs have MD cell lengths greater than or less than the MD cell lengths from non-like adjacent yarns from adjacent pairs. The present invention provides a solution to the problems of minimizing topographical and drainage markings resulting from warp crossover points and the arrangement of the left and right warps at the crossover points. This invention also minimizes the slippage between layers of the fabric.